The present invention relates to a system for limiting the smoke produced by a diesel engine during free or snap engine acceleration, that is, acceleration with the engine uncoupled or free from the vehicle drive line, without adversely affecting engine performance. More particularly, the modulator, which is located between the engine intake manifold boost pressure outlet and the fuel injection pump aneroid control, comprises a carefully tuned orifice and an accumulator defining a preselected volume for modulating the action of the injection pump aneroid to maintain fuel being supplied to the injection pump in full cutback during brief periods of free engine acceleration while allowing relatively normal operation of the aneroid control when accelerating a vehicle to maintain driveability.